1. Field of the Invention
The present invention relates to an optical-beam scanning apparatus and an image forming apparatus that has a function of an image magnification control.
2. Description of the Related Art
A technology of an image magnification control is disclosed in, for example, Japanese Patent Application Laid-open No. 2003-279873.
An optical-beam scanning apparatus in the above technology can correct dot positions with high precision. According to the optical scanning apparatus, two detectors respectively detect a start position and an end position of write effective region for a scanning laser beam. The detected positions are input to a dot-position deviation detector/controller. The dot-position deviation detector/controller measures time spent for the laser beam to scan between positions of the detectors to obtain a scanning time, and compares measured scanning time with a scanning time when ideal scanning has been performed. Thus, a deviation in the scanning time is obtained. Based on the deviation, phase data is corrected. The optical-beam scanning apparatus shifts the phase of a clock to correct magnification. Correction of magnification is commonly executed with a clock frequency of a phase locked loop (PLL).
If the correction is executed only by a PLL, however, to achieve accurate correction, the PLL needs to have a high accuracy. This results in a large circuit scale and a cost increase. If the correction is executed only by phase shifting, the phase is partially shifted. This results in deterioration of an image quality.